Implantable heart monitoring device and method

ABSTRACT

In an implantable heart monitoring device and method, particularly for monitoring diastolic dysfunction, a control circuit (a) detects the heart rate, (b) derives information correlated to the stroke volume of the heart at the detected heart rate, and (c) stores the detected heart rate and the derived information correlated to the stroke volume in a memory. The control circuit automatically implements (a), (b) and (c) at a number of different occasions for a number of different, naturally varying heart rates, so that the memory contains information indicating the stroke volume as a function of the heart rate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an implantable heart monitoring device,with which it is possible to monitor the heart condition. The inventionalso concerns a corresponding method.

2. Description of the Prior Art

Several different devices for monitoring the performance of a heart areknown. Often these devices are also able to deliver stimulation pulsesto the heart. The devices are often able to sense the electricalactivity in the heart. It is also known to determine an impedance valuemeasured between different electrodes positioned in or at the heart. Itis also known to sense other physiological parameters, such as pressure,oxygen level etc.

US 2001/0012953 A1 describes bi-ventricular pacing. An impedance may bemeasured between electrodes on the right and the left sides of theheart. The variation of the impedance with time is detected. Thedetected impedance variation may be used in order to synchronise thecontraction of the ventricles.

US 2001/0021864 A1 describes different manners of using the proximal anddistal electrodes of different leads in order to inject a current and tomeasure an impedance. The measured impedance value may be used in orderto maximise the cardiac flow.

US 2007/0049835 A1 relates to an implantable cardioverter-defibrillatoror pacemaker whose standard circuitry is used to trend a physiologicalcardiac parameter using intra-cardiac impedance measurements.

US 2007/0100249 A1 describes an implantable medical apparatus fordetecting diastolic heart failure, DHF. The apparatus includes circuitryfor determining, as the DHF parameter, the time duration of apredetermined phase of diastole.

US 2007/0055170 A1 describes a device for detecting the state of a hearton the basis of intracardial impedance measurement. The device has animpedance measuring unit as well as an analysis unit, which is connectedto the impedance measuring unit and is implemented to derive a cardiacfunction parameter from a time curve of the impedance ascertained usingthe impedance measuring unit. The analysis unit derives a cardiacfunction parameter characterizing the behaviour of a heart during thediastole.

U.S. Pat. No. 6,314,323 describes a heart stimulator in which thecardiac output is determined by measuring the systolic pressure.

The article “Hemodynamic Effects of Tachycardia in Patients withRelaxation Abnormality: Abnormal Stroke Volume Response as OverlookedMechanism of Dyspnea Associated with Tachycardia in Diastolic HeartFailure” by Dae-Won Sohn et al., Journal of the American Society ofEchocardiography, February 2007, pp. 171-176, describes a comparativestudy of two groups of individuals: healthy individuals and individualswith stable relaxation abnormality. The article describes how leftventricular pressure and stroke volume varies for the two groups whenthe heart is paced with 80 beats per minute and 120 beats per minute.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an implantable heartmonitoring device with which it is possible to detect or monitor thestatus of the heart of a patient who suffers from a heart failure, inparticular a diastolic dysfunction. A further object is to provide sucha device, which with quite simple means makes it possible to detect ormonitor the status of the heart condition of a patient who suffers fromsuch heart deficiency.

The above objects are achieved by an implantable heart monitoring devicehaving at least one memory and a control circuit configured tocommunicate with a number of implanted electrode and/or sensor membersadapted to be positioned in a heart or in relation to a heart of aliving being such that the control circuit, by means of these members,is able to detect the heart rate of the living being and to deriveinformation correlated to the stroke volume of said heart, wherein thecontrol circuit is configured to carry out the following steps:

a) detect the heart rate,

b) derive said information correlated to the stroke volume at saiddetected heart rate, and

c) store the detected heart rate and the derived information correlatedto the stroke volume in said memory,

wherein the control circuit is configured to automatically carry outsteps a) to c) at a number of different occasions at different,naturally varying, heart rates, such that the memory containsinformation that indicates the stroke volume as a function of the heartrate.

The information concerning the stroke volume as a function of the heartrate constitutes important information about the status of the heart. Inparticular, when a patient suffers from a diastolic dysfunction, thestoke volume tends to decrease more with increasing heart rate than fora patent without such dysfunction. The more the stroke volume decreaseswith increasing heart rate, the worse is the dysfunction in question.Since according to the present invention, information concerning thestroke volume as a function of the heart rate is obtained, the deviceaccording to the invention provides important information concerning thestatus of the heart. Furthermore, the device carries out the mentionedsteps automatically, which means that for example no physician needs tobe present for carrying out the steps. The device does not have to havea complicated construction in order to provide the information. Forexample, an implantable heart stimulation device can with simplemeasures be constructed to constitute a device according to theinvention.

It should be noted that with naturally varying heart rate is in thisdocument meant that the heart rate is the heart rate of a heart that atthe occasion in question is not paced at all by a heart stimulatingdevice, or, if the heart is paced, then the pacing pulses are deliveredin accordance with the intrinsic rate of the heart.

It should also be noted that as a measure of the heart rate, for examplethe duration of the heart beat can be used (since the duration of aheart beat is the inverse of the heart rate). The duration can bedetected, for example as the RR-interval, i.e. the time between twoR-waves. The heart rate may thus be determined for an individual heartbeat.

According to an embodiment of the invention, the control circuit isconfigured to carry out the steps for at least three different heartrates such that the information that indicates the stroke volume as afunction of the heart rate can be determined with a sufficiently highaccuracy.

Since the control circuit is configured to carry out the steps for atleast three different heart rates, the stored information concerningstroke volume as a function of the heart rate can be determined with ahigh accuracy. Preferably, the control circuit is configured to carryout the steps for more than three different heart rates, for example forat least 5 different heart rates. Thereby it can be determined with evenhigher accuracy how the stroke volume varies as a function of the heartrate. Consequently, very accurate information about the status of theheart can be obtained.

According to an embodiment, the control circuit is configured to carryout the steps for different heart rates, which differ from each othersuch that the highest heart rate is at least 10%, preferably at least25%, higher than the lowest heart rate. Thereby, the stoke volume as afunction of the heart rate can be determined for a relatively largevariation of the heart rate, which means that a very good informationconcerning the status of the heart is obtained.

According to a further embodiment, the control circuit is configured tocreate a warning message if the obtained information that indicates thestroke volume as a function of the heart rate fulfils a predeterminedcriterion.

The predetermined criterion may for example be that the information thatindicates the stroke volume as a function of the heart rate indicatesthat the stroke volume decreases when the heart rate increases. Anothercriterion may be that the information that indicates the stroke volumeas a function of the heart rate indicates that the stroke volumedecreases more than a predetermined amount when the heart rateincreases.

With such a warning message, for example the patient in question or aphysician can be alerted. The warning message may be any kind of warningmessage. The warning message may for example be stored in the memory.Such a warning message may for example be transferred in a wirelessmanner to an external device, located outside of the living being. Sucha warning message may be communicated to the living being in which thedevice is implanted or to a physician.

According to a further embodiment, the control circuit is configured to,for a certain heart rate, derive the information correlated to thestroke volume during a plurality of heart beats, such that a moreaccurate information correlated to the stroke volume at the heart ratein question is obtained than if the information is derived only duringone heart beat.

It should be noted that when it is said that the control circuit isconfigured to, for a certain heart rate, derive the informationcorrelated to the stroke volume during a plurality of heart beats, thisdoes not mean that the heart rate must be exactly the same for thenumber of heart beats. For example, the information correlated to thestroke volume for a certain heart rate may be derived for 10 differentheart beats with heart rates varying between 72 and 75. From thisprocedure it is possible to determine for example an average measure ofthe stroke volume, and an average heart rate within the interval 72 to75. Thereby a measure of the stroke volume at the heart rate in questionis obtained. Hereby a more accurate information correlated to the strokevolume at the heart rate in question is obtained than if the informationis derived only during one heart beat.

According to a further embodiment, the control circuit is configuredsuch that the number of different occasions takes place within a firsttime period.

The time period may for example be less than 24 hours, preferable lessthan 1 hour, for example less than 20 minutes.

According to a further embodiment, the control circuit is configuredsuch that the number of different occasions will also take place withinat least one second time period, at a later time than the first timeperiod, such that the memory comprises information that indicates thestroke volume as a function of the heart rate both within the first timeperiod and within said second time period, wherein said memory containsinformation as to whether the indicated stroke volume as a function ofthe heart rate has changed between said first and second time periods.

The first and second time periods should thus not overlap with oneanother. The second time period may for example take place at least 1day, or at least one week, after the first time period. By determiningthe stroke volume as a function of the heart rate during different timeperiods, it is possible to monitor how the heart condition has changedbetween these time periods.

According to a further embodiment, the control circuit is configured tocreate a warning message if the indicated stroke volume as a function ofthe heart rate has changed more than a predetermined amount between saidfirst and second time periods. For example, a warning message may becreated if the function determined in the second time period indicatesthat the diastolic dysfunction has become worse, for example indicatedby the fact that the stroke volume decreases more with increasing heartrate than during the first time period.

The warning message may be any kind of warning message, for example asexplained above.

According to a further embodiment, the device has a detector fordetecting the physical activity of the living being, and the controlcircuit is configured to be able to determine the occasions when thesteps are to be carried out in dependence on physical activity sensed bythe detector for detecting the physical activity.

Since, the control circuit is configured to select said number ofoccasions in dependence on the physical activity sensed by saiddetector, it is possible to obtain the information that indicates thestroke volume as a function of the heart rate within a relatively shorttime. With the help of the sensed physical activity, it is thus possibleto determine whether the physical activity of the living being changes,such that the heart rate is likely to change. It is thereby possible toobtain the information correlated to the stroke volume for differentheart rates in a relatively short time. For example, each of thementioned first and second time periods may be shorter than 20 minutes.

Alternatively, it is of course also possible for the device to directlymonitor the heart rate of the living being and to select the differentoccasions when suitable, different heart rates are detected, such thatthe information that indicates the stroke volume as a function of theheart rate is obtained.

According to a further embodiment, the control circuit is configured toderive the information correlated to the stroke volume of said heart bycarrying out an impedance measurement that indicates how the amount ofblood in the left ventricle of the heart varies with time, whichmeasurement thus creates a signal that varies with time, and therebyconstitutes a curve that indicates how the amount of blood in the leftventricle of the heart varies with time.

Such an impedance measurement is an advantageous, and relatively simple,manner of determining the information that is correlated to the strokevolume.

According to a further embodiment, the control circuit is configured toderive the information correlated to the stroke volume either from theaforementioned curve, from a filtered such curve, from a template basedon different such curves from different heart beats or from a templatebased on different filtered such curves from different heart beats.

The detected impedance curve may thus be filtered, for example in orderto filter out artefacts and in order to make the curve smoother. Atemplate can be created based on a measurement during a plurality ofheart beats. The template is thus a representative curve based onmeasurements for a plurality of heart beats. The template may thus be akind of average, or typical curve based on measurements for differentheart beats.

According to a further embodiment, said information correlated to thestroke volume is derived by considering one or both of the following:

a) an area defined by the curve, filtered curve or template,

b) a peak-to-peak value of the curve, filtered curve or template.

In for example these manners, a good indication of the stroke volume canbe obtained.

According to a further embodiment, the control circuit is configured todetermine whether the contraction of the ventricle at the occasion inquestion belongs to a special category, and if this is the case, thenthe control circuit is either configured not to store the derivedinformation in the memory, or to store the derived informationseparately in said memory such that the memory contains information thatthe derived information relates to the special category such that thestored information in the memory is distinguished from the informationobtained when the contraction does not belong to the special category.

According to a further embodiment, the control circuit is configured todetermine whether the contraction of the ventricle at the occasion inquestion belongs to any of a number of different special categories, anddepending on to which special category the contraction belongs, thecontrol circuit is either configured not to store the derivedinformation in the memory, or to store the derived informationseparately in the memory, depending on the category, such the storedinformation in the memory for the different special categories can bedistinguished from one another.

For example, the control circuit can be configured to be able to detectone or more of the following special categories (in addition to normalsinus rhythm):

a) whether the contraction of the ventricle at the occasion in questionis a premature ventricular contraction,

b) whether the contraction of the ventricle at the occasion in questionrelates to atrial fibrillation,

c) whether the contraction of the ventricle at the occasion in questionis a supraventricular extra-systole,

d) whether the contraction of the ventricle at the occasion in questionis caused by left ventricular pacing,

e) whether the contraction of the ventricle at the occasion in questionis caused by right ventricular pacing,

f) whether the contraction of the ventricle at the occasion in questionis caused by biventricular pacing.

For different such special categories, the heart may behave differently,such that the mentioned information that indicates the stroke volume asa function of the heart rate may depend on the type (or category) ofheart beat. For example, for heart beats that are premature ventricularcontractions (PVCs), the mentioned information that indicates the strokevolume as a function of the heart rate may change more when the heartcondition becomes worse than for heart beats that are not PVCs.Consequently, it is an advantage of the invention that the informationthat indicates the stroke volume as a function of the heart rate isstored separately for, for example, PVCs. The stored information basedon PVC beats may thus be more sensitive to a change in the heartcondition and will therefore be suitable to use in order to monitor achange in the heart condition.

Another aspect of the invention relates to a method of monitoring aheart of a living being with the help of an implantable heart monitoringdevice. The method includes the following steps:

a) detect the heart rate,

b) derive information correlated to the stroke volume of the heart atthe detected heart rate, and

c) store the detected heart rate and the derived information correlatedto the stroke volume,

wherein the method also includes the following steps:

carry out steps a) to c) at a number of different occasions atdifferent, naturally varying, heart rates, such that information thatindicates the stroke volume as a function of the heart rate is obtained.

Such a method provides advantages corresponding to those described abovein connection with the device according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically an implantable heart monitoring device withleads and electrodes positioned in or in relation to a heart.

FIG. 2 shows schematically a control circuit and a memory which arecomprised in the heart monitoring device.

FIG. 3 show a schematic example of measured impedance as a function oftime.

FIG. 4 shows schematically examples of how the stroke volume may vary asa function of the heart rate for different heart conditions.

FIG. 5 is a schematic flow chart illustrating a method according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows schematically an embodiment of an implantable heartmonitoring device 10 according to the invention. The device 10 has acasing 12. The device 10 also has a control circuit 14, which controlsthe operation of the device 10. The device 10 also includes a memory 15connected to the control circuit 14. Furthermore, the device 10 has anactivity sensor 16 for sensing how physically active the living beingthat carries the device 10 is. The sensor 16 is connected to the controlcircuit 14.

The device 10 has a connector portion 13, via which the device 10 can beconnected to different leads 20, 30, 40. According to this embodiment,there are three leads 20, 30, 40. However, the number of leads can alsobe more or less then three. The leads 20, 30, 40 are provided withelectrode surfaces 21, 22, 31, 32, 41, 42, 43. The electrode surfaces21, 31, 41 are so-called tip electrodes, while the other electrodesurfaces 22, 32, 42, 43 are so-called ring electrodes.

The device 10 is a heart monitoring device. However, the device 10 mayalso have means for pacing a heart. Furthermore, the device may bedesigned to also function as a defibrillator. It should be noted thatthe device may have many more components and functions which are normalfor such devices.

According to the present embodiment, the implantable heart monitoringdevice 10 is set up to be able to sense the electrical activity of theheart and to pace different heart chambers. In the shown embodiment, thelead 20 has been introduced into the right atrium RA such that theelectrode surfaces 21, 22 are positioned in this atrium. The lead 30 hasbeen introduced into the heart such that the electrode surfaces 31, 32are positioned in the right ventricle RV. The electrode surfaces 21, 22can thus be used to sense and pace the right atrium RA and the electrodesurfaces 31, 32 can be used to sense and pace the right ventricle RV. LArepresents the left atrium of the heart. According to this example, noelectrodes are positioned to pace the left atrium LA.

The lead 40 has been introduced via the right atrium RA and the coronarysinus such that the electrode surfaces 41, 42, 43 are positioned in avein next to the left ventricle LV. The different electrode surfaces 41,42, 43 can thus be used to pace and sense the left ventricle LV in amanner known to a person skilled in the art. In this example, the lead40 has three different electrode surfaces 41, 42, 43 which make itpossible to choose which electrode surfaces are to be used for sensingand pacing.

It is also well-known to a person skilled in the art that differentelectrode surfaces can be used for injecting a current and for sensing avoltage in order to measure an impedance across at least a portion ofthe heart. Also the casing 12 can be used for this purpose.

FIG. 2 shows schematically in particular the control circuit 14 in somemore detail. The control circuit 14 comprises a control portion 18 thatcontrols the operation of the control circuit 14. The control portion 18is connected to the above mentioned memory 15. Furthermore, as is knownto a person skilled in the art, the control circuit 14 may comprise asensing circuit 25 and a pacing circuit 27, which circuits are adaptedto be connected to the lead 20 in order to pace and sense the rightatrium RA. Moreover, a sensing circuit 35 and a pacing circuit 37 areadapted to be connected to the lead 30 in order to sense and pace theright ventricle RV. Furthermore, a sensing circuit 45 and a pacingcircuit 47 are adapted to be connected to the lead 40 in order to senseand pace the left ventricle LV. The different sensing and pacingcircuits are of course also connected to the control portion 18. Thecontrol circuit 14 may be designed such that it is possible to selectwhich of the electrode surfaces 21, 22, 31, 32, 41, 42, 43 that are tobe used. Of course, also the leads 20, 30 may be provided with more orless than two electrode surfaces.

The control circuit 14 is configured to operate in time cyclescorresponding to heart cycles. This is normal for an implantable heartmonitoring or pacing device.

The control circuit 14 is also configured to communicate with a numberof electrode surfaces 12, 21, 22, 31, 32, 41, 42, 43 and to measure animpedance with the help of at least two such electrode surfaces. Theimpedance indicates the impedance across a portion of the heart thatincludes at least a part of the left ventricle LV. How to measure suchan impedance is known to those skilled in the art, for example from someof the above-mentioned documents. For example, the control circuit 14can be configured to inject a current between the electrode surfaces 31and 41 and to measure a voltage between the electrode surfaces 32, 42.However, other combinations of electrode surfaces can be used for theimpedance measurement. However, the control circuit 14 is preferably setup such that the variation of the measured impedance is related to thevariation in the amount of blood in the left ventricle LV.

FIG. 3 shows schematically an example of how the measured impedance Zmay vary with time t. This schematic curve in FIG. 3 is rather smooth.Such a curve can be obtained by filtering the measured impedance. Theimpedance is in this case measured across the left ventricle LV. Theimpedance has a low value at the point 51 when the left ventricle LV isfilled with blood. The impedance increases thereafter until a maximumvalue 53 is obtained when the left ventricle LV contains a minimumamount of blood. Thereafter, the impedance decreases until a new minimumvalue 55 is obtained when the left ventricle LV is again filled withblood. The time t₁ between the minimum values 51 and 55 represents theduration of a heart beat. The heart rate is the inverse of this time t₁.The heart rate may for example be determined by means of theintracardial electrogram detected by the implanted device.

From the curve shown in FIG. 3 information correlated to the strokevolume SV can be obtained. As a measure of the stroke volume, forexample, the peak to peak value between the minimum 51 and the maximum53 can be used. Another alternative is to use an area defined by thecurve as a measure of the SV. The area can for example be defined as thearea between the curve and a predetermined base line, which for examplecan be the Z value indicated as 0 in FIG. 3. In this manner a value ofthe stroke volume SV for the heart rate in question can be determined.

In order to improve the measurement of the SV, it is possible todetermine a curve like the one shown in FIG. 3 for different heart beatsbut with essentially the same heart rate. From such different curves, atemplate may be formed that represents an average of the measured curvesfor the different heart beats at the heart rate in question.

The control circuit 14 is configured to detect the heart rate and toderive the mentioned information correlated to the stroke volume at thedetected heart rate. The control circuit 14 will store the detectedheart rate and the corresponding value correlated to the SV in thememory 15.

Furthermore, the control circuit 14 is configured to carry out thesesteps at a plurality of different heart rates, for example for at leastfive different heart rates. The highest heart rate may thereby be atleast 25% higher than the lowest heart rate. The different heart ratesshould be naturally varying heart rates as defined above. By storing theinformation related to the SV for the different heart rates in thememory 15, the memory 15 will comprise information that indicates thestroke volume as a function of the heart rate.

Such information is schematically illustrated in FIG. 4. This Figureshows the determined stroke volume SV as a function of the heart rate HRfor two different situations represented by the functions 57 and 59.Each dot represents the determined stroke volume for a certain heartrate. The function 57 may for example have been determined for a certainpatient within a first time period (for example within an hour a certainday). The function 59 may have been obtained for the same patient duringa second time period, for example a month later. Each function 57, 59thus represents the status of the heart at the time that the function inquestion was determined. For the function 57, the stroke volume does notdecrease much when the heart rate increases. However, for the function59, the stroke volume decreases more when the heart rate increases. Thisis an indication of the fact that the diastolic function of the heart isworse for the function 59 than for the function 57. The heart conditionof the patient in question has thus become worse.

Instead of storing all the measurement values in the memory 15, it ispossible to only store information that indicates the heart condition atthe time period in question. For example the slope of the function 57and 59, respectively, can be the stored value that represents the heartcondition.

The control circuit 14 may be configured to create a warning message,for example if for a certain function the stroke volume SV decreasesmore than a predetermined amount with increasing heart rate HR (if thenegative slope of the function is higher than a predefined value).

The control circuit 14 may be configured to determine the functions(like 57 and 59) during different time periods as exemplified above. Thecontrol circuit 14 may thereby be configured to create a warning messageif the indicated stroke volume as a function of the heart rate haschanged more than a predetermined amount between the different timeperiods. The warning message may for example be created if the heartcondition has become worse. For example, since the heart conditionrepresented by the function 59 is worse than the heart conditionrepresented by the function 57, this means that a warning message may becreated in this case.

In order to determine the occasions when the control circuit 14 is tocarry out the measurements in order to determine the stroke volume as afunction of the heart rate, the detector 16 for detecting the physicalactivity of the patient in question may be used. For example, thecontrol circuit 14 may detect that the patient is physically active suchthat the heart rate is likely to change. Thereby the measurement stepscan be carried out for the different heart rates such that a function asdescribed above can be determined within a relatively short time period.

Preferably, the control circuit 14 is configured to determine the kindof heart beat that is involved when the stroke volume at the heart ratein question is determined. The control circuit 14 can thereby beconfigured to categorize the contraction of the ventricle in differentcategories depending on the kind of contraction. The contraction may forexample be categorized as a normal sinus rhythm, as a prematureventricular contraction, as a contraction occurring during atrialfibrillation, as a supraventricular extra-systole or as a contractioncaused by pacing (left ventricular pacing, right ventricular pacing orbiventricular pacing). The control circuit 14 can thereby be configuredto store the derived information regarding the stroke volume at theheart rate in question separately in the memory 15 for the differentcategories of heart contractions. Thereby a function like the onesrepresented in FIG. 4 can be determined separately for the differentcategories of heart beats.

FIG. 5 shows a schematic flow chart illustrating a method according tothe invention. At the same time FIG. 5 illustrates how the device 10according to the invention can be set up to operate.

First it is determined when to carry out the measurement with the helpof the device 10. For example, it can be programmed into the device 10that the measurements in question are to be carried out once a week.

Next, an electrode configuration for carrying out the impedancemeasurement is determined. This can be programmed into the device inadvance, or the device 10 can automatically select an appropriateelectrode configuration for the measurement. For example, as indicatedabove, the measurement can be carried out by injecting a current betweenthe electrode surfaces 31 and 41 and by measuring a voltage between theelectrode surfaces 32 and 42.

Next, with the help of the detector 16, the physical activity of thepatient is detected in order to determine if it is appropriate to carryout a measurement. Alternatively, the exact moments for carrying out themeasurements may be determined in other manners. For example, the heartrate may be detected and if the heart rate is suitable to carry out themeasurement, a measurement may be carried out.

Thereafter the impedance is thus measured during a heart beat at acertain heart rate. Preferably, the impedance curve is determined for aplurality (for example at least five) of heart beats at a certain heartrate. A template may thus be created. This template constitutes arepresentative curve for the measured impedance during a heart beat atthe heart rate in question. The determined impedance curve, or thecorresponding value representing stroke volume, is stored together withthe heart rate.

As mentioned above, the stored information can be stored separately fordifferent categories, depending on the kind of heart contraction. Thedifferent heart contractions that are used to create a template for thecategory in question should thus be of the same kind.

The same procedure is carried out for different heart rates untilsufficient information is obtained in order to determine the strokevolume as a function of the heart rate. The stroke volume as a functionof the heart rate, or a corresponding value thereof, is stored. Thestored information is preferably stored separately for the differentcategories of heart contractions.

A warning message may be created if the information concerning thestroke volume as a function of the heart rate (for a certain category)fulfils a predetermined criterion. The criterion may for example be thatthe stroke volume decreases more than a predetermined amount when theheart rate increases.

The procedure described above is repeated at a later occasion, forexample a week later. Thereby new information is obtained that indicatesthe stroke volume as a function of the heart rate. A warning message maythen for example be created if the heart condition has become worse.

The warning message may for example be stored in the memory 15 in orderto be communicated to a device external of the patient. The warningmessage may for example be communicated to a physician. Of course, notonly the warning message but also other stored information may becommunicated to an external device.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventors to embody within thepatent warranted heron all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

We claim as our invention:
 1. An implantable heart monitoring devicecomprising: a control circuit having an input that receives a signalfrom at least one electrode that senses a parameter related to cardiacactivity; a memory in communication with said control circuit; and saidcontrol circuit being configured to (a) detect the heart rate of thesubject from said signal (b) derive information correlated to the strokevolume of the heart at the detected heart rate, and (c) store thedetected heart rate and the derived information correlated to the strokevolume in said memory, and said control circuit being configured toautomatically execute (a), (b) and (c) at a plurality of differentoccasions for different, naturally varying heart rates, to enterinformation into said memory to produce a representation in said memoryof the stroke volume as a function of the heart rate; wherein saidcontrol circuit is configured to operate said electrode to execute ameasurement as to how an amount of blood in the left ventricle of theheart varies with time, and to derive said information correlated to thestroke volume of the heart from a result of said measurement; whereinsaid control circuit is configured to generate a curve, as said resultof said measurement, indicating how said amount of blood in the leftventricle varies with time, and to derive said information correlatedwith the stroke volume from the curve or by an operation on said curveselected from the group consisting of filtering said curve, analyzing atemplate based on different curves for different heartbeats, andgenerating and analyzing a template based on filtering different curvesfor respectively different heartbeats; and wherein said control circuitis configured to derive said information correlated to the stroke volumefrom a characteristic of said curve, said filtered curve or saidtemplate, selected from the group consisting of an area and apeak-to-peak value.
 2. An implantable heart monitoring device as claimedin claim 1 wherein said control circuit is configured to execute (a),(b) and (c) for at least three different heart rates.
 3. An implantableheart monitoring device as claimed in claim 1 wherein said controlcircuit is configured to execute (a), (b) and (c) for different heartrates that differ from each other by at least 10% between a highest ofsaid different heart rates and a lowest of said different heart rates.4. An implantable heart monitoring device as claimed in claim 1 whereinsaid control circuit is configured to emit a warning signal if saidinformation entered into said memory indicates that said stroke volumeas a function of said heart rate satisfies a predetermined criterion. 5.An implantable heart monitoring device as claimed in claim 1 whereinsaid control circuit is configured to, for a predetermined heart rate,derive said information correlated to the stroke volume during aplurality of heartbeats.
 6. An implantable heart monitoring device asclaimed in claim 1 wherein said control circuit is configured to execute(a), (b) and (c) at a plurality of different occasions that occur withina predetermined time period.
 7. An implantable heart monitoring deviceas claimed in claim 6 wherein said predetermined time period is a firstpredetermined time period, and wherein said control circuit isconfigured to also execute (a), (b) and (c) during a plurality offurther different occasions that occur within a second predeterminedtime period, that follows said first predetermined time period, and toenter information into said memory indicating said stroke volume as afunction of said heart rate both within said first predetermined timeperiod and within said second predetermined time period, and whereinsaid control circuit is configured to identify whether a change in saidstroke volume as a function of said heart rate has occurred between saidfirst predetermined time period and said second predetermined timeperiod.
 8. An implantable heart monitoring device as claimed in claim 7wherein said control circuit is configured to emit a warning signal ifsaid change exceeds a predetermined amount.
 9. An implantable heartmonitoring device as claimed in claim 1 comprising a detector adapted todetect physical activity of said living subject, and wherein saidcontrol circuit is configured to determine said occasions for executing(a), (b) and (c) dependent on said physical activity.
 10. An implantableheart monitoring device as claimed in claim 1 wherein said controlcircuit is configured to determine whether, at any of said plurality ofoccasions, contraction of the ventricle is in a predetermined categoryand, if so, to modify (c) by a modification selected from the groupconsisting of not storing said detected heart rate and said derivedinformation correlated to the stroke volume in said memory, and storingsaid detected heart rate and the derived information correlated to thestroke volume in said memory with a designation indicating that thedetected heart rate and the derived information correlated to the strokevolume were obtained when said contraction was in said predeterminedcategory.
 11. An implantable heart monitoring device as claimed in claim1 wherein said control circuit is configured to determine whether, atany of said plurality of occasions, contraction of the ventricle is inany of a plurality of predetermined categories and, if so and dependingon the category, to modify (c) by a modification selected from the groupconsisting of not storing said detected heart rate and said derivedinformation correlated to the stroke volume in said memory, and storingsaid detected heart rate and the derived information correlated to thestroke volume in said memory with a designation indicating that thedetected heart rate and the derived information correlated to the strokevolume were obtained when said contraction was in said predeterminedcategory.